It has been shown that electromagnetic energy transfer between a hot and cold body is a function of the close spacing of the bodies due to evanescent coupling of near fields. Thus, the closer the bodies, approximately one micron and below, the greater the power transfer. For gap spacings of 0.1 microns, increases in the rate of energy transfer of factors of five and higher are observed.
The dilemma, however, is forming and then maintaining the close spacing between two bodies at a sub-micron gap in order to maintain enhanced performance. While it is possible to obtain the sub-micron gap spacing, the thermal effects on the hot and cold surfaces induce cupping, warping, or deformation of the elements resulting in variations in gap spacing thereby resulting in uncontrollable variances in the power output.
Typically, in order to increase power output, given the lower power density of prior art devices, it has been necessary to increase the temperature. Temperature increases, however, are limited by the material of the device and system components.
Micron gap thermal photovoltaic (MTPV) systems are a potentially more efficient way to use photovoltaic cells to convert heat to electricity. Micron gap thermal photovoltaic devices are an improved method of thermal photovoltaics which is the thermal version of “solar cell” technology. Both methods make use of the ability of photons to excite electrons across the bandgap of a semi-conductor and thereby generate useful electric current. The lower the temperature of the heat source, the narrower the bandgap of the semi-conductor must be to provide the best match with the incoming spectrum of photon energy. Only those photons with energy equal to or greater than the bandgap can generate electricity. Lower energy photons can only generate heat and are a loss mechanism for efficiency. A preferred micron gap thermal photovoltaic system would include a source of heat radiated or conducted to an emitter layer which is suspended at a sub-micron gap above the surface of an infrared sensing photovoltaic cell.
By using a sub-micron gap between a hot emitting surface and a photovoltaic collector, a more enhanced rate of transfer of photons from solid to solid is observed than is possible with large gaps. Additional transfer mechanisms are involved other than simply Planck's law of the radiation, although the spectral distribution of the photons is that of a black body. The use of sub-micron gaps, however, implies that a vacuum environment is used to avoid excessive heat conduction across the gap by low energy photons that cannot excite electrons into the conduction band. To make efficient use of the source of heat, a high fraction of high energy photons must be generated. The structure used to separate the emitting surface from the photovoltaic cell must be both small in diameter and also a very good thermal insulator for the same efficiency considerations. The photovoltaic cell will generally have to be cooled somewhat so that it will function properly. At high temperatures, intrinsic carrier generation swamps the PN junction and it is no longer an effective collector of electrons.
Micron gap thermal photovoltaic systems function as though the emitter has an emissivity value greater than one. The definition of a black body is that it has an emissivity value equal to one and this value cannot be exceeded for large gap radiant energy transfer. Equivalent emissivity factors of 5-10 have been experimentally demonstrated by the Assignees hereof using gaps in the region of 0.30 to 0.10 microns.
There are at least two ways to take advantage of this phenomenon. In a comparable system, if the temperature of the emitting surfaces is kept the same, the micron gap thermal photovoltaic system can be made proportionately smaller and cheaper while producing the same amount of electricity. Or, if a comparable size system is used, the micron gap thermal photovoltaic system will run at a considerably lower temperature thereby reducing the cost of materials used in manufacturing the system. In a preliminary estimate, it was calculated that by using micron gap technology the operating temperature of a typical system could be reduced from 1,400° C. to 1,000° C. and still produce the same output of electricity. Such a lowering of temperature could make the difference in the practicality of the system due to the wider availability and lower cost of possible materials.
Additional energy transfer mechanisms have been postulated and the ability to build systems using narrow thermally isolated gaps may find use in many types of applications in accordance with the subject invention.
In the Assignee's previous patent, U.S. Pat. No. 6,232,546, incorporated herein by this reference, an array of emitters was supported on a panel by flexures and the top surface of each emitter spaced from the photovoltaic element or cell by spacers formed on each emitter. The micron gap thermal photovoltaic devices constructed to date according to the disclosure of the '546 patent have typically been small chips four square millimeters in area and not particularly well suited for the generation of sufficient electrical power in some applications.
The initial multi-piece construction method of achieving small gaps was to build thin, small diameter spacers on an emitter “heater chip” structure, clean the photovoltaic cell and the heater chip, and then place them together in an attempt to eliminate dirt particles or waviness in the surface which would limit achieving the desired small gap. This manufacturing technique could be difficult to accomplish on large area devices with dimensions on the order of inches or even feet.